An in vitro approach for studying organ and species differences in the activation of chemical carcinogens is being utilized. Intact cells are used for metabolic activation because they simulate in vivo metabolism. To assess biological activity of metabolites, the genetic endpoints, toxicity, mutation and/or SCE induction in V79 cells and reversion of S. typhimurium, are used. In addition, PHLC analysis of metabolites formed by primary cells from different organs and species have been conducted. Work during the past year has investigated differences among rat, dog, and bovine liver and bladder cell activation/metabolism of aromatic amines. Species differences in the relative ability of these organs to active aromatic amines have been observed. Results from another study indicated differences between rat and hamster hepatocyte activation of nitrosamines and aromatic amines and the relative sensitivities of V79 cell mutation and SCE induction and Salmonella reversion to the activated intermediates. Additionally, structure-activity analyses of short chain aliphatic nitrosamines in the hamster hepatocyte-mediated V79 cell mutagenesis system have been conduced and the possible correlation of mutagenicity with carcinogenicity investigated.